Your brain works together with the rest of your body in lots of different ways. If things go wrong with some organs, like if your heart stops pumping, your brain’s obviously going to have a bad time.

But sometimes the connection is a little less straightforward. Take the liver, for example: you need it for things like filtering blood, storing vitamins, and lots of other processes that keep you alive. So, basically, what happens in the liver doesn’t stay in the liver.

And an issue there can cause a chain reaction across your body that seriously affects your brain. Patients with many kinds of liver damage or disease can develop a condition called hepatic encephalopathy, or HE. Hepatic refers to the liver, and encephalopathy means brain disease, damage, or malfunction.

Physicians think of HE as a spectrum of symptoms that can affect personality, movement, cognition, and even levels of consciousness. They can range from mild, like mood swings, to severe, like a coma. And HE is mainly caused by substances that build up when the liver stops working as well as it should.

In a healthy liver, chemicals from the rest of the body are filtered out of the blood and broken down by specialized cells and enzymes. If the liver becomes damaged, like from certain drugs, a bad diet, or some other kind of injury or disease, the tissue changes structure and gets all scarred. This scar tissue isn’t able to function like normal liver cells.

Plus, all those structural changes make it harder for blood to get in for detoxification. And in really severe cases, a lot of blood may bypass the liver entirely. One important toxin your liver deals with is ammonia, which mostly comes from the digestion of proteins in your gut.

Ammonia gets turned into urea, which is excreted in pee. So a damaged liver can lead to a buildup of ammonia in the bloodstream, which is bad news for the brain. Ammonia can cross the blood-brain barrier, which is a membrane that blocks a lot of molecules in your bloodstream from circulating around your brain, to protect it from potential damage.

Once ammonia slips past, it’s mostly taken up by astrocytes, which are brain cells that help out neurons in a lot of different ways. And astrocytes have enzymes that convert ammonia into the amino acid glutamine to help protect the neurons. If they do this too much, though, studies have found that their structure gets messed up.

Cell-damaging molecules called free radicals get produced, and their mitochondria stop working as efficiently, which means they can’t make as much energy. As the astrocytes lose function, they stop getting rid of ammonia, and everything gets even more messed up. Too much ammonia can affect the storage, production, and use of neurotransmitters across the whole brain.

In other words, your neurons can’t really communicate, which can cause a whole range of problems with mood, movement ... pretty much anything. Even though ammonia is a significant part of HE, other toxins might be involved too. And researchers are still figuring out exactly what they are and how they might make HE worse.

Besides waste products like ammonia, your liver also has a huge impact on how long lots of drugs— from aspirin to LSD— stay in your system before they’re flushed out. A healthy liver takes drug molecules from your bloodstream and converts them into different chemicals. Sometimes they’re the active form of a drug, but eventually they become more water-soluble compounds that can be peed out.

To do this, it mostly uses a system of enzymes known as the cytochrome p450 family, which may be responsible for up to 75% of drug metabolism. So if a disease damages liver cells or directly interferes with these enzymes, things can go sour. Even just as we age naturally, the activity of cytochrome p450 enzymes seems to dip.

Slower drug metabolism can mean a normally fine painkiller, like acetaminophen, may build up until it hits toxic levels. This can lead to more damage, and eventually we’re right back to hepatic encephalopathy. Now some brain diseases are linked to toxic substances that some people intentionally put in their bodies, which their livers have to process—like ethanol from alcoholic drinks.

Basically, in the process of breaking down ethanol, the byproducts can cause tissue damage and trigger inflammation. And if your liver keeps getting damaged, it won’t be able to handle ethanol as well. It’s a vicious cycle.

It turns out that too much ethanol can affect your cells’ ability to take up, store, and use thiamine, also known as vitamin B1, from your diet. Some enzymes that depend on thiamine are a key part of glycolysis, a process your body uses to break down glucose molecules and make energy. So a lack of thiamine can limit the amount of energy your body can get from food.

And since the brain is basically an energy-hungry monster, that is a recipe for disaster. And it can lead to Korsakoff syndrome, one of the lesser-known diseases that affects memory. Patients find themselves basically frozen in time, with retrograde and anterograde amnesia.

So they can’t recall large chunks of their past, or form new memories going forward. Specifically, in patients’ MRIs, we’ve seen shrinking in the medial thalamus, mammillary bodies, and nearby areas—which are all linked with memory. With treatments aimed at getting thiamine and other nutrient levels back to normal, many Korsakoff’s patients can at least partly recover.

But, basically, if you want to keep your brain in tip-top condition, remember to take good care of the rest of you as well. And take special care of your liver—just a good ol' ... wherever it is. Thanks for watching this episode of SciShow Psych!

If you want to keep learning about the human brain with us, from questions about behavior to more deep dives into disease, you can go to youtube.com/scishowpsych to subscribe! [OUTRO ♪].